N-15 ENRICHMENT IN AGRICULTURAL CATCHMENTS - FIELD PATTERNS AND APPLICATIONS TO TRACKING ATLANTIC SALMON (SALMO-SALAR)

Nitrogen isotopes were used to study the source of nitrate and its upt ake into the food web of a northeastern river in Vermont, USA. In six tributaries of the White River (Vermont) nitrate concentrations were e levated in streams flowing through areas with agricultural land use as compared to streams flowing through pristine forested areas. We obser ved a strong positive correlation between delta(15)N values of stream water nitrate and percent of agricultural land-use within a given catc hment. Agricultural sites had relatively high delta(15)N values of nit rate (+7.3 parts per thousand) compared to forested sites (+2.0 parts per thousand). These relatively high delta(15)N values coupled with re latively high concentrations of nitrate in agricultural streams sugges t the introduction of N-15-enriched nitrate draining from agricultural areas. Additionally, elevated delta(15)N values of algae, aquatic ins ects, and Atlantic salmon (Salmo salar) in agricultural sites compared to lower delta(15)N values of their counterparts in forested sites su ggest the uptake of nitrate from agricultural sources by stream organi sms. We observe a similar positive correlation between the delta(15)N of Atlantic salmon and the percent of agricultural land in a catchment . The nitrogen isotope values of both Atlantic salmon and water nitrat e reflect land-use. Thus, nitrogen isotope ratios in salmon, especiall y when coupled with strontium isotope ratios (which reflect catchment geology), create unique isotopic identities for Atlantic salmon stocki ng streams and thereby provide a means of determining the natal origin of juvenile salmon. (C) 1998 Elsevier Science B.V. All rights reserve d.